Bone fixation devices are used in orthopedic surgery to align or fix a predetermined relationship between adjacent bones. One particular use for bone fixation devices is the fixation and alignment of adjacent vertebral bodies. These fixation devices commonly include an elongate rod to set the vertebrae into a desired alignment and various anchoring devices, such as hooks, bolts, wires, screws, and the like that secure the rods to the bone.
A common vertebral fixation device employs a rod and screw system. These systems may include plates that are attached to adjacent vertebrae desired to be fixed in order to provide a mounting base for the fixation device on each vertebrae. A bone screw may then be inserted through openings in each mounting plate and screwed into each adjacent bone. To fix the position of the vertebrae, a spinal rod is oriented to extend between the fixation devices and is secured to the bone screws by lockable connecting members.
The mounting plate is advantageously employed to impair or minimize toggling of the bone screws when the rod and screws are subjected to a load. Severe or constant toggling of the bone screws can weaken the attachment to the bone, which may require replacement of the fixation device. Current mounting devices are in the form of staples that include multiple, narrow and discrete spikes that are driven into the bone. Generally, these prong or spike projections have depending shanks with a generally cylindrical, curved, or otherwise relatively small-sized peripheral surface having narrow flats that extend from the staple to pointed or sharp-edged ends for penetrating the bone material. These peripheral surfaces only present a small or curved surface area in confronting relation to adjacent bone material. The confronting surface portions, be they curved or flat, offer very little in the way of resistance thereto. Accordingly, subjecting these small sized shank surfaces to extensive cyclic loading such as when attached to vertebrae in a vertebral fixation device generally will cause weakening of the connection to the vertebral surface and over time cause loosening and play to develop at the interface between the surfaces and bone material. With loose play at the staple, there is the undesirable potential that the bone screw will be able to shift and toggle with loading applied thereto.
Most current vertebral staples having multiple spikes also have a shortcoming in that they generally do not allow the position of the staple to be adjusted after the staple is implanted into the bone. If first implanted in an incorrect position, the staple must be removed and re-inserted. A vertebral staple disclosed in WO 2006/025921 attempts to overcome this shortcoming by having shorter perimeter spikes and a longer, central spike. The shorter perimeter spikes allow rotation of the staple body when the longer central spike is only partially driven into the bone along a small end portion of its length. Therefore, such rotation permits final positioning of the screw openings of the spike prior to the final insertion where both the long and short spikes are driven into the bone.
While the staple of the '921 publication addresses the positioning shortcomings of other vertebral staples, this staple still employs several discrete spikes having relatively small periphery extending about the shanks. The narrow bone confronting surface portions of the perimeter and central spikes of the staple in the '921 publication still exhibit the same shortcomings of other vertebral staples where movement of the plate over time is more likely when experiencing repeated loads.
Accordingly, there is a need for a mounting device for use in a bone fixation system that allows pre-positioning of the implant prior to final implantation, and provides a stable mounting base for the fixation system.